GB794201A - Improvements in or relating to methods and apparatus employing photomultipliers - Google Patents

Abstract

794,201. Copying telegraphy and television. TIME, Inc. April 15, 1955 [April 15, 1954], No. 10985/55. Class 40 (3). A complex electrical oscillation for the transmission of picture telegraphy or television signals comprising a high-frequency carrier modulated in accordance with the intensity of light incident upon a photo-multiplier is produced by applying to one dynode an alternating voltage which is substantially free from second harmonic and induces anode current variations at a frequency which is the second harmonic of the frequency of the applied voltage, and filtering the anode current so as to pass signals at the current variation frequency and side-band frequencies thereof and to reject the frequency of the voltage applied to the selected dynode. Apparatus for producing such an oscillation comprises a photo-multiplier having a cathode 19; anode 20 and several dynodes 21-29, the photo-multiplier operating with low plate current (e.g. 0.2 mA.). In order to prevent the anode current due to light variations being swamped by leakage currents, dynode 23 has applied thereto an alternating voltage free of second harmonic. This voltage is generated by an amplifier tube 70 and resonant circuits 71, 72 and 73, 74 tuned to a single frequency and is applied to the dynode 23 through an inductance 75, shunted by variable condenser 76 and inductively coupled to the coil 74 of resonant circuit 73, 74. The applied voltage has a peak amplitude substantially equal to the voltage difference between adjacent dynodes and induces a current variation of twice the frequency constituting a carrier which is modulated by the light variations. This modulated carrier and a signal, due to leakage and having the frequency of the applied alternating voltage, appears at the anode and the output is then filtered so as to pass only the modulated carrier. The filter comprises two coils 85, 86 coupled through shielded cable 87 and having an adjustable condenser 88 in series to vary the band width. The inductance 75 is also connected through variable resistor 80 and variable condenser 81 so as to provide an alternating voltage at anode 20 which largely neutralizes the voltage due to leakage.